Crude oil and natural gas production often involves generating produced water along with the petroleum products. Reusing this water, either as part of the petroleum production, or for hot water and steam generation, for agriculture and irrigation uses, or for municipal water supply is important to maintain an efficient and responsible petroleum production process. Membrane purification systems are known for purifying water by rejecting contaminants in the produced water down to very low levels, including those that meet or exceed domestic and industrial water requirements.
However, the conventional methods for using RO (reverse osmosis) membrane systems for water purification require careful and extensive pretreatment of the produced water prior to purification in the membrane purification system. Produced water frequently contains oil, solid particulates and high amounts of hardness, all of which easily cause scaling issues in a membrane purification system. Membrane scaling quickly degrades membrane separation performance; it is one of the continuing factors that prevent further commercial employment of membrane purification systems for water purification.
In conventional methods, the produced water undergoes an extensive pretreatment process to remove contaminants in the produced water that would otherwise cause membrane scaling. At a minimum, conventional pretreatment methods include reducing or removing hardness from the produced water prior to RO membrane purification. Water softening methods are known; these are often employed to remove the dissolved solids in the produced water that easily form scale on RO membranes. Examples include calcium, magnesium and barium compounds of oxide, carbonate and sulfate. In additional, convention purification processes include adjusting the pH of produced water prior to the RO membrane purification, to further reduce the chances of scaling occurring. Levels of pH of 10 and higher are known and taught. While adding sufficient alkali to the produced water to achieve this high pH value (at a significant cost and at additional operating complexity), difficulties arise with separating some of the contaminants from the produced water. For example, boron and ammonia are preferentially removed from produced water at different pH values. Operating the membrane purification system at a high pH may not even be effective for achieving a desired level of water purification.
There is a need for improved methods for purifying produced water.